Continuous mode solder jet apparatus and method

ABSTRACT

A solder jet apparatus and method is disclosed. The solder jet apparatus is a continuous mode solder jet that includes a blanking system and raster scan system. The use of the raster scan and blanking systems allows for a continuous stream of solder to be placed anywhere on the surface in any desired X-Y plane. This allows for greater accuracy as well as greater product throughput. Additionally, with the raster scan system, repairs to existing soldered surfaces can be quickly and easily performed using a map of the defects for directing the solder to the defects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/616,184,filed Jul. 7, 2003, pending, which is a continuation of application Ser.No. 10/159,000, filed May 30, 2002, now U.S. Pat. No. 6,588,645, issuedJul. 8, 2003, which is a continuation of application Ser. No.09/924,525, filed Aug. 8, 2001, now U.S. Pat. No. 6,443,350, issued Sep.3, 2002, which is a continuation of application Ser. No. 09/569,215,filed May 11, 2000, now U.S. Pat. No. 6,325,271, issued Dec. 4, 2001,which is a continuation of application Ser. No. 09/388,032, filed Sep.1, 1999, now U.S. Pat. No. 6,082,605, issued Jul. 4, 2000, which is acontinuation of application Ser. No. 08/989,578, filed Dec. 12, 1997,now U.S. Pat. No. 5,988,480, issued Nov. 23, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to applying solder to asubstrate and, more particularly, to the selected placement of solderusing a solder jet.

Depositing solder selectively on a substrate is well known. Differenttechniques include stenciling or screening a solder paste onto thesubstrate, using solder balls selectively placed where metal contact isdesired, and chemically vapor depositing the metal onto the surface ofthe substrate. Each one of these methods has advantages anddisadvantages.

The use of a stencil to fabricate a conductive trace pattern on thesurface allows for precise alignment and placement of the solder.Unfortunately, the stencils are expensive to design and produce and theywear out after repeated use. When they wear out, solder seeps throughthe worn stencil areas across those areas where no solder is desired,causing shorts, or no solder is being placed where it is needed, causinga breach or open connection. These areas have to be repaired and ifthese types of conditions are repeated with any type of frequency, thestencil must be replaced with a new stencil. Additionally, stencilsrequire periodic cleaning, which adds another processing step to cleanthe stencil as well as lessens the useful life of the stencil.

The use of solder balls has been a tremendous advance in the art ofelectrically connecting a device to the surface of a printed circuitboard. Solder balls, however, have quality control problems as theircritical dimensions continue to decrease. The ability to produce ballsof the same diameter consistently decreases as the diameter decreases.Thus, for some diameters of solder balls, the range of acceptableproduct can be solder balls having diameters more than twice the desireddiameter. Or, they can have diameters half the size of the desireddiameter. This requires that the tolerances at the surface contact levelof a substrate, such as a semiconductor device, must allow for a solderball having a diameter that is from 50% smaller to 100% larger than thespecified size. Further, working with solder balls is difficult becauseof their size and the methods needed to place them accurately. When theyfail to be placed accurately, or are missing entirely, problems occur inthe resulting assembly of a semiconductor device attached to a substratethat must be corrected. These problems include shorts or opens that mustbe fixed. No easy solution yet exists for repairing missing orimproperly sized solder balls after a semiconductor device has beenmechanically attached in place on a substrate.

Chemical vapor deposition (CVD) allows for precise alignment ofconductive traces and for batch processing. CVD does have limitationshowever. These limitations include being unable to place the packagedirectly on the surface of the printed circuit board (PCB) immediatelyafter depositing the metal on the surface since a cooling step istypically needed. Further, clean conditions are always necessary whenusing CVD, which requires expensive equipment and control. Additionally,when clean conditions do not exist, shorts or opens in assemblies canoccur that need to be repaired once they are discovered.

A new approach to deposit solder on a surface, such as a printed circuitboard (PCB), is to deposit the solder using a solder jet, similar to themanner in which ink jets deposit ink onto paper for printing. The inkjet technology is well established, but due to different problemsassociated with solder, ink jet technology is not directly applicable tosolder jet technology. For example, solder jets use molten melt as aprint agent, whereas ink jets use heated water-based ink. Since theprint agent is metal in solder jets, the viscosities and densities aremuch different as are the operating temperatures. Thus, applying ink jetsolutions to solder jet problems is impractical.

One typical solder jet apparatus has recently been developed by MPMCorporation. The solder jet apparatus takes liquid solder and forms itinto a stream of droplets that have a uniform size and composition. Theformation of the droplets involves generating a consistent pressurecoupled with a vibration force sufficient enough to dislodge the dropsfrom the jet nozzle in a steady state with a uniform size andconsistency. Once the solder droplets are formed, gravity forces themdownward where they impact on the surface of the substrate. The solderdroplets pass through a charging electrode to impart a charge on themetal droplets.

The system operates using a binary control that either allows thedroplets to impact on the surface or to be removed into a dropletcatcher for recycling when no droplets are desired. Since the dropletswere charged at one point, an electric field or pulse can be asserted,causing the droplets to either continue to the surface or to fall intothe catcher. With this system, the exact position of the droplets isknown and never varies. Thus, the substrate must be moved to the desiredgrid for the droplets to impact the area desired to be soldered. Thisresults in a highly inefficient system since the substrate must bestopped for each application of solder to a new location. This alsoinvolves greater mechanical complexity since the table holding thesubstrate, or the solder jet apparatus itself, must be moved and alignedproperly before solder can be deposited.

Accordingly, what is needed is a solder applicator that allows forgreater precision in placing the droplets along with increasedefficiency in product throughput.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, a solder jet apparatus is disclosed.The solder jet apparatus is a continuous mode solder jet that includes ablanking system and raster scan system. The use of the raster scan andblanking systems allows for a continuous stream of solder to be placedanywhere on the surface in any desired X-Y plane. This allows forgreater accuracy as well as greater product throughput. Additionally,with the raster scan system, repairs to existing soldered surfaces canbe quickly and easily performed using a map of the defects for directingthe solder to the defects.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a solder jet apparatus accordingto the present invention; and

FIG. 2 is a top plan view of a substrate having solder depositedaccording to the solder jet apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A solder jet apparatus 10 is depicted in the schematic block diagram ofFIG. 1. Solder jet apparatus 10 deposits metal on substrate 12 in theform of solder droplets 14. The solder droplets 14 can be directed in anX-Y plane of deflection using a raster scan and blanking system. Thisallows the solder droplets to be “written” on substrate 12.

The solder droplets 14 are formed from melted metal held in liquid metalreservoir 16. A temperature controller 18 is connected to liquid metalreservoir 16 so that the temperature of the liquid metal held in thereservoir can be kept at a desired temperature that leads to optimumdroplet formation and release. For example, the solder eutectictemperature at the point of release is 190° C. and its temperature atimpact is 183° C. To prevent solder droplets 14 from cooling too rapidlyor from oxidizing, a constant surrounding temperature is provided and,if desired, the apparatus can be placed in a container that is eitherunder vacuum or is filled with an inert gas.

The solder droplets 14 can be formed by the application of a drivingpressure and a sufficient vibration force. The driving pressure can beprovided by pressure inducer 20, which is comprised of a piezoelectriccrystal that is driven by a tuning frequency sufficient enough to causepressure to build up in liquid metal reservoir 16. The mechanicalvibration is generated by vibrator 22, which comprises a secondpiezoelectric crystal that is driven by another tuning frequency,causing liquid metal reservoir 16 to vibrate. The timing of the pressureand the vibrations is established so as to produce uniform droplets ofthe same consistency. Once the solder droplets 14 are formed, thevibration releases them from liquid metal reservoir 16 and the force ofgravity draws them down at a predictable velocity.

Liquid metal reservoir 16 further includes a solder jet nozzle 23, whichis opened and closed via a solenoid 24. The aperture of solder jetnozzle 23 is selected with a size sufficient enough to generate thedroplets of a desired size. The solder droplets 14 are formed having adiameter of micron size, ranging from 40-300. When solenoid 24 isactivated, it either closes or opens solder jet nozzle 23.

Solder droplets 14 pass through several zones before either beingdeposited on substrate 12 or recycled back to liquid metal reservoir 16.The first zone is a charging field driven by charge driver 26. Chargedriver 26 causes charge electrodes 28 to generate an electric fieldtherebetween. As solder droplets 14 pass past charge electrodes 28, theyare imparted with an electric charge. With this charge, solder droplets14 can be deflected at later stages as appropriate.

The second zone is a blanking zone that uses blanking electrodes or coil30. The blanking electrodes are activated having sufficient electricfield so as to cause solder droplets 14 to deflect to a catcher 32. Thisis the return function of the scanning function as is described below.Catcher 32 catches the liquid solder and causes the metal to be recycledto liquid metal reservoir 16. This prevents solder droplets 14 fromdepositing on the surface of substrate 12. This blanking can be done ina selective manner so that droplets are deposited in some locations, butnot others. Blanking electrodes or coil 30 are controlled by signalcontroller 34. Signal controller 34 can be a signal processor such as acomputer system. The computer system allows greater control of solderdroplets 14 by programming the blanking electrodes or coil 30 to turn onand off in a desired sequence so as to pattern the substrate with adesired solder pattern. An alternative embodiment can include an air jetsystem if the electrical pulse is insufficient to remove the droplets. Aphoto cell can be located above the air jet system in order to ensureproper timing of electrical pulses or the air pressure.

The third zone is the raster scan system and includes electrostaticdeflection plates or magnetic coil 36. Electrostatic deflection plates36 are charged by signal controller 34 so that solder droplets 14 aredeflected in either the horizontal X-direction or the verticalY-direction, or both. Further, the solder droplets 14 can be held in asteady position in the X-Y plane in order to build up the solder to adesired height. Since the droplet stream now scans in the X- andY-directions, the substrate 12 can now stay stationary throughout thedroplet application process. Signal controller 34 can be programmed toperform a variety of soldering patterns for placing solder droplets 14on substrate 12. For example, a CAD/CAM system programmed with a desiredoutput sends signals to blanking electrodes 30 and to electrostaticdeflection plates 36 to guide the droplet stream in the desired patternof placing droplets in certain locations, but not in others.Additionally, when the “stream” of solder droplets 14 is returned to thebeginning of the horizontal scan, blanking electrodes 30 cause thesolder droplets 14 to deflect to catcher 32 so as not to “write” acrossthe substrate during the return scan. The location of blankingelectrodes 30 and electrostatic deflection plates 36 can be switched, ifdesired.

An electronic light sensor 38, which connects to signal controller 34,is positioned so that the solder droplets 14 pass through the electroniclight sensor 38. Electronic light sensor 38 is used to count the numberof solder droplets 14 passing by. This allows signal controller 34 tomonitor the droplet output and either blank or pass droplets as needed.

FIG. 2 is a top plan view of the surface of substrate 12 as solderdroplets 14 are deposited. A first line 40 scans across the surface,depositing solder droplets 14 in selected positions and leaving blanks42 in the remaining positions. A return scan line 44, which is ghosted,indicates when the stream of droplets is caught by catcher 32 as thestream returns to the beginning of the next line 40. This process isrepeated as often as is necessary with catcher 32 collecting all theblank spots and scan returns. Alternatively, solenoid 24 can beactivated to close solder jet nozzle 23 during the return scan. Thisalso prevents unwanted solder droplets 14 from depositing on the surfaceof substrate 12.

The type of solder used with the solder jet apparatus 10 can include anytype of metal solder such as, for example, 63/37 PbSN, 62/36/2PbSnAa,In/Sn.

The solder jet apparatus 10 can be used for many types of solderapplication. One type of application includes that of applying uniformsolder balls, in the form of solder droplets, to the substrate 12. Thisprovides a universal ball applicator system. Further, the system canrepair particular locations where the solder ball application processhas failed to insert a desired solder ball. In order to repair any andall solder ball defects, a scan of the surface of substrate 12 can beprovided and then a map of the defective areas can be programmed to thesignal controller 34. This allows for a rapid repair of the surface ofsubstrate 12 where solder balls had been omitted. Another application isto pre-tin a location on substrate 12. Pre-tinning is accomplished byapplying one or more droplets to the same location or to apply dropletsin such a manner as to thoroughly cover the surface of substrate 12 or agrid section of substrate 12.

Similar to pre-tinning is pre-plating a board. Pre-plating a boardinvolves applying solder droplets over the entire surface area of theboard to cover it with a metal plate. An exposed portion of the boardcan be selected where desirable. Typically, this area is along the edgeof the board either on one edge, two edges, or all four edges, or can bein the center section of the board. Prior methods of pre-plating a boardresulted in a problem known as “measling.” Measling is where small holesexist in the plating surface that lead to electrical defaults. The useof the solder jet apparatus 10 allows the system to eliminate themeasling locations by applying solder directly to those openings.Additionally, using the pre-plating process provided by solder jetapparatus 10 eliminates measling entirely. Just as pre-plated boards mayhave measling problems, boards that had been stenciled with solder pastehad similar problems. These problems can include openings or gaps in thestenciled design. Again, a map of the surface defects can be ascertainedand then used by the signal controller 34 to make appropriate correctionand repair to those particular problem points. Additionally, large areascan be printed using the X-Y motion of the table in combination with theX-Y slowing of the solder application. Also, the final ball size can bechanged on demand. Further, in prior ball application systems that apply7 balls/sec, the board needs to be moved to a new location. With thisinvention, no relocation time is required, thus reducing processingtime.

While the present invention has been described in terms of certainpreferred embodiments, it is not so limited, and those of ordinary skillin the art will readily recognize and appreciate that many additions,deletions and modifications to the embodiments described herein may bemade without departing from the scope of the invention as hereinafterclaimed.

1. A liquid metal deposition apparatus comprising: a continuous streamgenerator for producing a stream of liquid metal solder droplets, theliquid metal solder droplets having a substantially uniform size withina consistent predetermined range, the consistent predetermined range ofsubstantially uniform size metal solder droplets being within a size ofa selected bond pad of one a semiconductor die and the contact pads ofthe substrate; and a director for selectively directing a stream ofliquid metal solder droplets after being produced by the continuousstream generator onto the selected bond pads of the at least onesemiconductor die of the substrate, the stream director including araster scanner for scanning the stream of liquid metal solder droplets,the raster scanner including: an electrical charge generator forcharging at least a portion of the liquid metal solder droplets of thestream of liquid metal solder droplets with an electrical charge; astream blanking device for intermittently blanking at least some of theliquid metal solder droplets of the stream of liquid metal solderdroplets; and an electrically charged droplet deflector for deflectingat least one electrically charged liquid metal solder droplet of thestream of liquid metal solder droplets in a first direction and a seconddirection for deposition at a location of a plurality of locationsextending throughout the surface of the substrate while the substrateremains stationary.
 2. The apparatus according to claim 1, wherein thecontinuous stream generator comprises: a pressure inducer; and thevibrator comprises a vibrator connected to the pressure inducer forcausing formation of the stream of liquid metal solder droplets inconnection with the pressure inducer.
 3. The apparatus according toclaim 2, wherein the pressure inducer comprises a piezoelectric crystaloperating at a desired frequency.
 4. The apparatus according to claim 2,wherein the vibrator comprises a piezoelectric crystal operating at aselected frequency to form liquid metal droplets having a size in therange of micron size droplets of a liquid metal solder.
 5. The apparatusaccording to claim 1, wherein the continuous stream generator includes asolder jet nozzle having an aperture producing a consistent range ofdroplets of the liquid metal solder for forming the stream of liquidmetal solder droplets.
 6. The apparatus according to claim 5, whereinthe continuous stream generator further includes a solenoid connected tothe solder jet nozzle.
 7. The apparatus according to claim 1, whereinthe stream blanking device at least provides blanking of the at leastsome of the stream of liquid metal solder droplets when the stream ofliquid metal solder droplets is positioned between the endpoint of afirst location of the plurality of locations extending throughout thesurface of the substrate and the start point of a second location of theplurality of locations extending throughout the surface of thesubstrate.
 8. The apparatus according to claim 1, wherein the streamblanking device comprises: a deflector field device selectivelydeflecting at least one droplet of the stream of liquid metal solderdroplets; and a droplet catcher catching the at least one droplet whichhas been deflected from the stream of liquid metal solder droplets priorto the at least one droplet which has been deflected from the stream ofliquid solder droplets being deposited on at least one bond pad of theat least one semiconductor die of the substrate.
 9. The apparatusaccording to claim 1, wherein the stream director includes aprogrammable direction controller for determining a direction of thestream of liquid metal solder droplets.
 10. A method for depositingliquid metal comprising: producing a stream of liquid metal solderdroplets having a substantially uniform size within a consistentpredetermined range, the consistent predetermined range of substantiallyuniform size metal solder droplets being within a size of a selectedbond pad of one a semiconductor die and the contact pads of thesubstrate; and directing a stream of liquid metal solder droplets afterbeing produced by the continuous stream generator onto the selected bondpads of the at least one semiconductor die of the substrate, the streamdirector including a raster scanner for scanning the stream of liquidmetal solder droplets, the raster scanner including: an electricalcharge generator for charging at least a portion of the liquid metalsolder droplets of the stream of liquid metal solder droplets with anelectrical charge; a stream blanking device for intermittently blankingat least some of the liquid metal solder droplets of the stream ofliquid metal solder droplets; and deflecting at least one electricallycharged liquid metal solder droplet of the stream of liquid metal solderdroplets in a first direction and a second direction for deposition at alocation of a plurality of locations extending throughout the surface ofthe substrate while the substrate remains stationary using anelectrically charged droplet deflector.
 11. The method according toclaim 10, wherein said producing further comprises: heating a soldermetal to a liquid state in the reservoir; and controlling a temperatureof the liquid solder metal in the reservoir for providing said stream ofliquid solder metal droplets in said liquid state while selectivelydirecting said stream of liquid solder metal droplets for contactingportions of a substrate located on a stationary support.
 12. The methodaccording to claim 10, wherein said producing further comprises:inducing a pressure on a source of liquid metal; and vibrating saidliquid metal to cause said stream of liquid solder metal droplets to beformed as said pressure is induced on said source of liquid metal. 13.The method according to claim 12, wherein said pressure inducing isgenerated by a first piezoelectric crystal driven by a given frequencyto produce a desired pressure.
 14. The method according to claim 13,wherein said vibrating is generated by a second piezoelectric crystaldriven by a selected frequency to produce a given vibration frequencysufficient enough to form droplets having a diameter in the range of 40microns to 300 microns.
 15. The method according to claim 10, whereinsaid producing further comprises forming said stream of liquid soldermetal droplets having a consistent diameter in the range of 40 micronsto 300 microns.
 16. The method according to claim 10, wherein saidblanking comprises blanking when said stream of liquid solder metaldroplets is positioned between an endpoint of a first horizontal scanline and a start point of a second horizontal scan line with respect toa substrate located on a stationary support.
 17. The method according toclaim 10, wherein said blanking further comprises: deflecting saidstream of liquid solder metal droplets; and catching said deflectedstream of liquid solder metal droplets to prevent said drops from beingdeposited on said substrate located on a stationary support.
 18. Themethod according to claim 10, wherein said directing comprisesprogrammably controlling a direction of said stream of liquid soldermetal droplets for deposition on portions of a substrate located on astationary support.
 19. The method of claim 10, wherein the directingsaid stream of liquid solder metal droplets in a first dimension and asecond dimension, such that solder is deposited at said locations on asubstrate maintained in a stationary portion located on said stationarysupport, said directing comprising: raster scanning said stream ofliquid solder metal droplets, said raster scanning includingelectrically charging said stream of liquid solder metal droplets;electrostatically deflecting said electrically charged stream of liquidsolder metal droplets in a first variable electrostatic potential insaid first dimension for contacting portions of a substrate located on astationary support; electrostatically deflecting said electricallycharged stream of liquid metal droplets in a second variableelectrostatic potential in said second dimension to said locations onsaid substrate located on a stationary support; and blanking selectivelysaid stream of liquid solder metal droplets to prevent a portion of saidstream of liquid solder metal droplets from contacting said substratelocated on a stationary support.